The deposition of coatings on a substrate is conventionally accomplished using either a plasma torch which creates a plasma plume that envelopes substrates disposed on the inner wall of a tube anode, such as in the device disclosed in Moll U.S. Pat. No. 4,555,611, or an electric arc generated between a cathode and an anode which creates an ionized plasma stream in the vicinity of the substrate. Devices incorporating these methods are well known to those skilled in the art.
Disadvantages to any apparatus of this type are the lack of uniformity of plasma distribution in the anode area and the limited length of the reaction zone. In the case of for example a plasma jet or plume, the reaction zone is limited by the input power of the plasma source. For example, in the microwave tube furnace described by Martin and Hill in "Novel Synthesis Routes for Diamond Films in a Heated Flowtube", Proceedings of the Second International Conference on New Diamond Science and Technology, Sep. 23-27, 1990, pp.365-70, microwave discharge is used to improve excitation and dissociation of the reaction species in a tube furnace. The length of the reaction zone is limited by the input power of the microwave source.
In the case of an arc-generated plasma stream, such as that described in European Patent No.478,909 Al to Balzers Aktiengesellschaft, the homogeneous reaction zone is limited to the stable portion of the arc, and the ionized plasma tends to be denser in the cathode region. In both cases this severely limits the size of the substrate that can be coated and the quality or uniformity of the coating. Additionally, because the arc is dispersed throughout the apparatus and not well confined, in order to simultaneously process multiple substrates, the deposition process must be carried out at low pressures, reducing plasma commensurately reducing the efficiency and rate of deposition.
In the case of arc jet CVD processing, as for example the apparatus and process described by Boudina et al in Thermal Diffusivity of Diamond Films Synthesized from Methane by Arc Discharge Plasma Jet CVD, Diamond and Related Materials, 2(1993) 852-858, the reaction zone is limited by the cross sectional area of the jet.